Thin aluminium flakes

ABSTRACT

The present invention relates to aluminium flakes having a thickness between 7 and 9 nm, coating compositions containing them and the use of the coating compositions for forming (security) products. When the compositions of the present invention are used in coating a hologram the obtained products show an extremely bright OVD image and extremely strong rainbow effect, high purity and contrast.

The present invention relates to aluminium flakes having a thicknessbetween 7 and 9 nm, coating compositions containing them and the use ofthe coating compositions for forming (security) products. When thecompositions of the present invention are used in coating a hologram theobtained products show an extremely bright OVD image and extremelystrong rainbow effect, high purity and contrast.

GB-A-1,465,908 (U.S. Pat. No. 4,116,710) relates to a method for thepreparation of particulate metal, such as aluminium, which methodcomprises depositing a metal coating on a substrate by vapor,electroless or sputter deposition and removing the metal coating fromthe substrate by dissolving the substrate in a solvent therefor, thethickness of the deposited coating being such that upon dissolution ofthe substrate the metal is released as a plurality of metal particles.The metal platelets obtained by the process have a thickness of from 20to 100 nm, especially 35 to 60 nm.

U.S. Pat. No. 4,321,087 discloses a continuous process for preparingfinely divided metal particles comprising the steps of: (a) applying arelease coating onto at least one side of a continuous carrier sheet inan amount of from 0.75 to 1.50 lbs. of said release coating per side ofsaid carrier sheet, (b) depositing in the form of a thin film, a metalselected from the group consisting of aluminum, chromium, copper, steel,silver and gold, in an amount of from 35 to 45 nm thickness directlyonto said release coating, (c) passing said carrier sheet with saidrelease coating and said film of said metal through a solvent forsolubiling said release coating but which is non-reactive with saidmetal, (d) removing said film of said metal from said carrier sheet in aparticulate form to produce the metal particles substantially free ofsaid release coating, and collecting the metal particles in anon-reactive solvent which is non-reactive with said metal. (e)concentrating the metal particles, and (f) breaking the metal particlesinto pigment particles having a particle size diameter between about 25and 50 microns.

WO0024946 discloses a process for making flakes comprising: providing avapor deposition chamber; placing a transport device in the vapordeposition chamber; providing a release coat source and a vacuumdeposition source in the vacuum deposition chamber directed toward thetransport device, in which the deposition source deposits a layer offlake material; applying a vacuum to the chamber, and while the chamberis evacuated, applying-alternate layers of a release coat from therelease coat source and a vapor deposited flake layer from the vacuumdeposition source to the transport device in sequence to build up amulti-layer sandwich of alternating flake material layers andintervening release coat layers, the release coat layers comprising adissolvable material that forms a smooth continuous barrier layer andsupport surface on which the flake material layers can be formed, sothat removal of the sandwich from the evacuated chamber yields amulti-layer sandwich which can be easily separated into flakes of fineparticle size by subsequent treatment with a material that essentiallycompletely dissolves the intervening release coat layers to remove themfrom the flakes.

According to WO0024946 another process for making metal flakes is aprocess of Avery Dennison Corporation for making flakes sold under thedesignation Metalure®. In this process both sides of a polyester carrierare gravure coated with a solvent-based resin solution. The dried coatedweb is then transported to a metallizing facility where both sides ofthe coated sheet are metallized by a thin film of vapor depositedaluminum. The sheet with the thin metal film is then returned to thecoating facility where both sides of the aluminum are coated with asecond film of the solvent-based resin solution. The dried coated/metalsheet is then transported again to the metallizing facility to apply asecond film of vapor deposited aluminum to both sides of the sheet. Theresulting multi-layer sheet is then transported for further processingto a facility where the coatings are stripped from the carrier in asolvent such as acetone. The stripping operation breaks the continuouslayer into particles contained in a slurry. The solvent dissolves thepolymer out from between the metal layers in the slurry. The slurry isthen subjected to sonic treatment and centrifuging to remove the solventand the dissolved coating, leaving a cake of concentrated aluminumflakes approximately 65% solids. The cake is then let down in a suitablevehicle and further sized by homogenizing into flakes of controlled sizefor use in inks, paints, and coatings. Metal flakes produced by thisprocess for use in printable applications such as inks are characterizedby a particle size from about 4 to 12 microns and a thickness from about150 to about 250 angstroms.

WO020090613 as well as WO03046245 discloses a process for thepreparation of flakes having a high aspect ratio in which the flakeshave an average particle size from about 4 to about 12 microns and asingle layer thickness from about 5 to about 500 angstroms.

WO02/094945 relates to a method for the production of plane-parallelplatelets, comprising the steps:

a) vapour-deposition, at a pressure below atmospheric pressure, of aseparating agent onto a carrier to produce a separating agent layer,b) vapour-deposition, at a pressure below atmospheric pressure, of atleast one product layer onto the separating agent layer, andc) dissolution of the separating agent layer in a solvent and productionof a suspension in which the at least one product layer is present inthe form of plane-parallel platelets, in which method the separatingagent is selected from the group consisting of anthracene,anthraquinone, acetamidophenol, acetylsalicylic acid, camphoricanhydride, benzimidazole, benzene-1,2,4-tricarboxylic acid,biphenyl-2,2-dicarboxylic acid, bis(4-hydroxyphenyl)sulfone,dihydroxyanthraquinone, hydantoin, 3-hydroxybenzoic acid,8-hydroxyquinoline-5-sulfonic acid monohydrate, 4-hydroxycoumarin,7-hydroxycoumarin, 3-hydroxynaphthalene-2-carboxylic acid, isophthalicacid, 4,4-methylene-bis-3-hydroxynaphthalene-2-carboxylic acid,naphthalene-1,8-dicarboxylic anhydride, phthalimide and its potassiumsalt, phenolphthalein, phenothiazine, saccharin and its salts,tetraphenylmethane, triphenylene, triphenylmethanol, and also mixturesof at least two of those substances. The planparall platelets obtainedby said process have typically a thickness of 30 to 500 nm and adiameter of from 5 to 50 μm.

WO06/021528 relates to a process for the production of plane-parallelplatelets, comprising the steps:

a) vapour-deposition of a separating agent onto a carrier to produce aseparating agent layer,b) vapour-deposition of at least one product layer onto the separatingagent layer, andc) dissolution of the separating agent layer in a solvent and productionof a suspension in which the at least one product layer is present inthe form of plane-parallel platelets, wherein the separating agent isselected from the group consisting of anthracene, anthraquinone,acetamidophenol, acetylsalicylic acid, camphoric anhydride,benzimidazole, benzene-1,2,4-tricarboxylic acid,biphenyl-2,2-dicarboxylic acid, bis(4-hydroxyphenyl)sulfone,dihydroxyanthraquinone, hydantoin, 3-hydroxybenzoic acid,8-hydroxyquinoline-5-sulfonic acid monohydrate, 4-hydroxycoumarin,7-hydroxycoumarin, 3-hydroxynaphthalene-2-carboxylic acid, isophthalicacid, 4,4-methylene-bis-3-hydroxynaphthalene-2-carboxylic acid,naphthalene-1,8-dicarboxylic anhydride, phthalimide and its potassiumsalt, phenolphthalein, phenothiazine, saccharin and its salts,tetraphenylmethane, triphenylene, triphenylmethanol, and also mixturesof at least two of those substances, especially pentaerythritol(C(CH₂OH)₄), trimesic acid (=1,3,5 benzene tricarboxylic acid),DL-alanine, DL-valine, 2,6-diaminopurine, ascorbic acid,1,3,5-benzenetricarboxylic acid, o-acetylsalicyclic acid, diphenic acid,terephthalic acid, pyrogallol, cyanuric acid, hexamethyltetramine(urotropin), fumaric acid, and 4-acetylbenzoic acid and also mixtures ofat least two of those substances.

WO07/057,328 discloses a process for the production of plane-parallelplatelets, comprising the steps:

a) deposition of a separating agent I, which is dissolvable in water,onto a carrier to produce a separating agent layer,b) vapour-deposition of a separating agent II, which is not dissolvablein water, onto the separating agent layer of step a),c) vapour-deposition of at least one product layer onto the separatingagent layer of step b), andd) vapour-deposition of a separating agent II, which is not dissolvablein water, onto the product layer of step c),e) dissolution of the separating agent layer of step a) in water andproduction of a suspension in which the at least one product layer ispresent in the form of plane-parallel platelets, the top surface and thebottom surface, but not the side surfaces of which are covered by theseparating agent II, andf) dissolution of the separating agent layer of steps b) and d) in asolvent and production of a suspension in which the product, comprisingat least one layer, is present in the form of plane-parallel platelets.

The aluminium flakes described in WO06/021528 and WO07/057,328 have anaverage diameter of at least 2 μm, especially from 2 to 20 μm, moreespecially from 3 to 15 μm, and most preferred from 5 to 15 μm. Thethickness of the aluminium flakes is generally from 10 to 150 nm,especially from 10 to 100 nm, and more especially from 30 to 60 nm.

WO2005/051675 relates to a method for forming a (security) productcomprising the steps of:

-   a) providing a sheet of base material, said sheet having an upper    and lower surface;-   b) depositing a metallic ink on at least a portion of the sheet of    base material; and-   c) forming a diffraction grating on at least a portion of the    metallic ink, wherein the optical density of metallic ink when    deposited is in the range of 0.2 to 0.8.

The average pigment particle diameter is in the range 8-15 μm and thethickness of the pigment particles is in the range 10-50 nm, especially19-21 nm.

WO2005/049745 discloses a coating composition for use in coating adiffraction grating, comprising metal pigment particles and a binderwherein the ratio of pigment to binder is sufficiently high as to permitthe alignment of the pigment particles to the contours of thediffraction grating. Preferably, the thickness of the pigment particlesis less than 50 nm. More preferably, the thickness of pigment particleis less than 35 nm. More preferably still, the thickness of pigmentparticle is less than 20 nm. Even more preferably still, the thicknessof pigment particle is in the range 5-18 nm. In one embodiment ofWO2005/049745, the thickness of the pigment particles is in the range10-50 nm. In another embodiment, the thickness of pigment particle is inthe range 10-30 nm. In another embodiment, the average thickness ofpigment particle is 17 nm. In another embodiment, the average thicknessof pigment particle is 12.5 nm.

It is the object of the present invention to provide aluminium flakeswhich, when used in coating a hologram result in products showing anextremely bright OVD image and extremely strong rainbow effect, highpurity and contrast.

Said object has been solved by aluminium flakes having a thickness above5 nm and below 10 nm.

Preferably, the aluminium flakes have a thickness between 7 and 9 nm.

The mean particle diameter may be in the range of 2 to 20 μm. The meanparticle diameter is preferably in the range of 5 to 20 μm, morepreferably 8 to 15 μm, even more preferably 9 to 10 μm as measured by alaser diffraction instrument (Coulter LS130).

The optical density may be in the range of 0.16 to 0.24 as measured onthe McBeth densitometer. Preferably, the range is 0.18 to 0.22. Morepreferably, the optical density is 0.2 as measured on the McBethdensitometer.

The aluminium flakes of the present invention can be used for thepreparation of coating compositions. Accordingly, the present inventionrelates also to coating compositions, comprising the aluminium flakes ofthe present invention.

The coating compositions comprise the aluminium flakes of the presentinvention and a binder. The ratio of pigment to binder is sufficientlyhigh as to permit the alignment of the pigment particles to the contoursof the diffraction grating.

The binder is a high-molecular-weight organic compound conventionallyused in coating compositions. The high molecular weight organic materialfor the pigmenting of which the pigments or pigment compositionsaccording to the invention may be used may be of natural or syntheticorigin. High molecular weight organic materials usually have molecularweights of about from 10³ to 10⁸ g/mol or even more. They may be, forexample, natural resins, drying oils, rubber or casein, or naturalsubstances derived therefrom, such as chlorinated rubber, oil-modifiedalkyd resins, viscose, cellulose ethers or esters, such asethylcellulose, cellulose acetate, cellulose propionate, celluloseacetobutyrate or nitrocellulose, but especially totally syntheticorganic polymers (thermosetting plastics and thermoplastics), as areobtained by polymerisation, polycondensation or polyaddition. From theclass of the polymerisation resins there may be mentioned, especially,polyolefins, such as polyethylene, polypropylene or polyisobutylene, andalso substituted polyolefins, such as polymerisation products of vinylchloride, vinyl acetate, styrene, acrylonitrile, acrylic acid esters,methacrylic acid esters or butadiene, and also copolymerisation productsof the said monomers, such as especially ABS or EVA.

Advantageously, the coating composition further comprises a solvent.

The binder may comprise any one or more selected from the groupcomprising nitrocellulose, ethyl cellulose, cellulose acetate, celluloseacetate propionate (CAP), cellulose acetate butyrate (CAB), alcoholsoluble propionate (ASP), vinyl chloride, vinyl acetate copolymers,vinyl acetate, vinyl, acrylic, polyurethane, polyamide, rosin ester,hydrocarbon, aldehyde, ketone, urethane, polythyleneterephthalate,terpene phenol, polyolefin, silicone, cellulose, polyamide, polyesterand rosin ester resins.

A colour change can be obtained by changing the pigment/binder ratio. Alow pigment/binder ratio (1:10 to 1:20) results in a deep bluish greycolour, whereas a higher pigment/binder ratio (1:1 to 1:0.5) gives ayellowish gold colour. For pigment/binder ratios inbetween the colourcan be seen to gradually shift from bluish grey to yellowish gold.

The coating composition is preferably a printing ink. The ink accordingto the present invention comprises, as in the case of an ordinaryprinting ink, the aluminium pigment, a binder, an auxiliary agent, andthe like.

With respect to the binder resin, a thermoplastic resin may be used,examples of which include, polyethylene based polymers [polyethylene(PE), ethylene-vinyl acetate copolymer (EVA), vinyl chloride-vinylacetate copolymer, vinyl alcohol-vinyl acetate copolymer, polypropylene(PP), vinyl based polymers [poly(vinyl chloride) (PVC), poly(vinylbutyral) (PVB), poly(vinyl alcohol) (PVA), poly(vinylidene chloride)(PVdC), poly(vinyl acetate) (PVAc), poly(vinyl formal) (PVF)],polystyrene based polymers [polystyrene (PS), styrene-acrylonitrilecopolymer (AS), acrylonitrile-butadiene-styrene copolymer (ABS)],acrylic based polymers [poly(methyl methacrylate) (PMMA), MMA-styrenecopolymer], polycarbonate (PC), celluloses [ethyl cellulose (EC),cellulose acetate (CA), propyl cellulose (CP), cellulose acetatebutyrate (CAB), cellulose nitrate (CN)], fluorin based polymers[polychlorofluoroethylene (PCTFE), polytetrafluoroethylene (PTFE),tetrafluoroethylene-hexafluoroethylene copolymer (FEP), poly(vinylidenefluoride) (PVdF)], urethane based polymers (PU), nylons [type 6, type66, type 610, type 11], polyesters (alkyl) [polyethylene terephthalate(PET), polybutylene terephthalate (PBT), polycyclohexane terephthalate(PCT)], novolac type phenolic resins, or the like. In addition,thermosetting resins such as resol type phenolic resin, a urea resin, amelamine resin, a polyurethane resin, an epoxy resin, an unsaturatedpolyester and the like, and natural resins such as protein, gum,shellac, copal, starch and rosin may also be used.

Furthermore, to the binder, a plasticizer for stabilizing theflexibility and strength of the print film and a solvent for adjustingthe viscosity and drying property thereof may be added according to theneeds therefor. The solvent may comprise any one or more of an ester,such as n-propyl acetate, iso-propyl acetate, ethyl acetate, butylacetate; an alcohol, such as ethyl alcohol, industrial methylatedspirits, isopropyl alcohol or normal propyl alcohol; a ketone, such asmethyl ethyl ketone or acetone; an aromatic hydrocarbon, such as xyleneand toluene. A solvent of a low boiling temperature of about 100° C. anda petroleum solvent of a high boiling temperature of 250° C. or higher,may be used according to the type of the printing method. Analkylbenzene or the like, for example may be used as a solvent of a lowboiling temperature. Examples of solvents are ethoxypropanol,methylethylketon, methoxypropylacetate, diacetonalcohol etc.

Further in addition, an auxiliary agent including a variety of reactiveagents for improving drying property, viscosity, and dispersibility, maysuitably be added. The auxiliary agents are to adjust the performance ofthe ink, and for example, a compound that improves the abrasionresistance of the ink surface and a drying agent that accelerates thedrying of the ink, and the like may be employed.

A photopolymerization-curable resin or an electron beam curable resinwherein a solvent is not used may also be employed as a binder resinthat is a principal component of the vehicle. The examples thereofinclude an acrylic resin, and specific examples of acrylic monomerscommercially available are shown below.

A monofunctional acrylate monomer that may be used includes for example,2-ethylhexyl acrylate, 2-ethylhexyl-EO adduct acrylate, ethoxydiethyleneglycol acrylate, 2-hydroxyethyl acrylate, 2-hydroxypropyl acrylate,2-hydroxyethyl acrylate-caprolactone adduct, 2-phenoxyethyl acrylate,phenoxydiethylene glycol acrylate, nonyl phenol-EO adduct acrylate,(nonyl phenol-EO adduct)-caprolactone adduct acrylate,2-hydroxy-3-phenoxypropyl acrylate, tetrahydrofurfuryl acrylate,furfuryl alcohol-caprolactone adduct acrylate, acryloyl morpholine,dicyclopentenyl acrylate, dicyclopentanyl acrylate,dicyclopentenyloxyethyl acrylate, isobornyl acrylate,(4,4-dimethyl-1,3-dioxane)-caprolactone adduct acrylate,(3-methyl-5,5-dimethyl-1,3-dioxane)-caprolactone adduct acrylate, andthe like.

A polyfunctional acrylate monomer that may be used includes hexanedioldiacrylate, neopentyl glycol diacrylate, polyethylene glycol diacrylate,tripropylene glycol diacrylate, neopentyl glycol hydroxypivalatediacrylate, (neopentyl glycol hydroxypivalate)-caprolactone adductdiacrylate, (1,6-hexanediol diglycidyl ether)-acrylic acid adduct,(hydroxypivalaldehyde-trimethylolpropane acetal) diacrylate,2,2-bis[4-(acryloyloxydiethoxy)phenyl]propane,2,2-bis[4-(acryloyloxydiethoxy)phenyl]methane, hydrogenated bisphenolA-ethylene oxide adduct diacrylate, tricyclodecanedimethanol diacrylate,trimethylolpropane triacrylate, pentaerithritol triacrylate,(trimethylolpropane-propylene oxide) adduct triacrylate,glycerine-propylene oxide adduct triacrylate, a mixture ofdipentaerithritol hexaacrylate and pentaacrylate, esters ofdipentaerithritol and lower fatty acid and acrylic acid,dipentaerithritol-caprolactone adduct acrylate, tris(acryloyloxyethyl)isocyanurate, 2-acryloyloxyethyl phosphate, and the like.

Inks comprising the above resins are free of solvent and are soconstituted as to polymerize in chain reaction upon irradiation by anelectron beam or electromagnetic waves.

With respect to inks of ultraviolet-irradiation type among these inks, aphotopolymerization initiator, and depending on the needs therefor, asensitizing agent, and auxiliary agents such as a polymerizationinhibitor and a chain transfer agent, and the like may be added thereto.

With respect to photo-polymerization initiators, there are, (1) aninitiator of direct photolysis type including an arylalkyl ketone, anoxime ketone, an acylphosphine oxide, or the like, (2) an initiator ofradical polymerization reaction type including a benzophenonederivative, a thioxanthone derivative, or the like, (3) an initiator ofcationic polymerization reaction type including an aryl diazonium salt,an aryl iodinium salt, an aryl sulfonium salt, and an aryl acetophenonesalt, or the like, and in addition, (4) an initiator of energy transfertype, (5) an initiator of photoredox type, (6) an initiator of electrontransfer type, and the like. With respect to the inks of electronbeam-curable type, a photopolymerization initiator is not necessary anda resin of the same type as in the case of the ultraviolet-irradiationtype inks can be used, and various kinds of auxiliary agent may be addedthereto according to the needs therefor.

The inks comprise a total content of aluminum pigment of from 0.1 to 20%by weight, preferably 0.1-10% by weight based on the total weight of theink.

Preferably, the binder comprises 50% nitrocellulose in conjunction withany above mentioned resin.

The composition may additionally comprise a solvent. The solvent may beester/alcohol blends and preferably normal propyl acetate and ethanol.More preferably, the ester/alcohol blend is in a ratio of between 10:1and 40:1, even more preferably 20:1 to 30:1.

The solvent used in the metallic ink may comprise any one or more of anester, such as n-propyl acetate, iso-propyl acetate, ethyl acetate,butyl acetate; an alcohol, such as ethyl alcohol, industrial methylatedspirits, isopropyl alcohol or normal propyl alcohol; a ketone, such asmethyl ethyl ketone or acetone; an aromatic hydrocarbon, such astoluene, and water.

The average particle diameter of the aluminium flakes may be in therange of 2 to 20 μm. The average particle diameter is preferably in therange of 5.0 to 15.0 μm, more preferably 8 to 15 μm, even morepreferably 7 to 11 μm in diameter as measured by a laser diffractioninstrument (Coulter LS130).

Preferably, the composition is used in the manufacture of a hologram.Reference is made to WO2005/051675 and WO2008/061930. The methoddescribed therein for forming an optically variable image (an opticallyvariable device), especially a holographic diffraction grating, on asubstrate comprises the steps of:

-   a) forming an optically variable image (OVI) on a discrete portion    of the substrate; and-   b) depositing the metallic ink of the present invention on at least    a portion of the diffraction grating.

Preferably, the method comprises the steps of:

-   a) applying a curable compound to at least a portion of the    substrate;-   b) contacting at least a portion of the curable compound with OVI    forming means;-   c) curing the curable compound and-   d) depositing the metallic ink of the present invention on at least    a portion of the cured compound.

Referring to FIG. 1 of WO08/061930, paper, aluminium, or another opaquesubstrates (1) is printed with an ultra violet curable lacquer (2) onits lower surface. An optically variable device or other lens orengraved structure is cast (3) into the surface of the lacquer (2) witha clear shim (4) having the optically variable device or other lens orengraved structure thereon. The optically variable device or other lensor engraved structure image is imparted into the lacquer and instantlycured (6) via an UV lamp disposed through the shim (4) at normalprocessing speeds through polarizing lens (8), quartz roller (6), andclear polycarbonate roller (5). The optically variable device or otherlens or engraved structure image is a facsimile of the image on theclear shim. Metallic ink (9) is printed (10) over the optically variabledevice or other lens or engraved structure and causes the opticallyvariable device or other lens or engraved structure to become lightreflective. Further colours (11) can be subsequently conventionallyprinted in-line at normal printing process speeds. In an alternativeembodiment, the paper, aluminium, and all manner of other opaquesubstrate (1) is replaced with a filmic substrate. Such material issubstantially transparent and therefore the image is visible from bothsides of the surface.

The (security) product obtainable by using the above method is new andforms a further subject of the present application.

In a preferred embodiment of the present invention a coloured, ormetallic ink is deposited on a substrate, on which the opticallyvariable image is formed; before forming the optically variable image onat least a portion of the coloured, or metallic ink.

The substrate may comprise any sheet material. The substrate may beopaque, substantially transparent or translucent, wherein the methoddescribed in WO08/061,930 is especially suited for substrates, which arefor opaque to UV light (non-transparent). The substrate may comprisepaper, leather, fabric such as silk, cotton, tyvac, filmic material ormetal, such as aluminium. The substrate may be in the form of one ormore sheets or a web.

The substrate may be mould made, woven, non-woven, cast, calendared,blown, extruded and/or biaxially extruded.

The substrate may comprise paper, fabric, man made fibres and polymericcompounds. The substrate may comprise any one or more selected from thegroup comprising paper, papers made from wood pulp or cotton orsynthetic wood free fibres and board. The paper/board may be coated,calendared or machine glazed; coated, uncoated, mould made with cottonor denim content, Tyvac, linen, cotton, silk, leather,polythyleneterephthalate, polypropylene propafilm, polyvinylchloride,rigid PVC, cellulose, tri-acetate, acetate polystyrene, polyethylene,nylon, acrylic and polytherimide board. The polythyleneterephthalatesubstrate may be Melienex type film orientated polypropylene (obtainablefrom DuPont Films Willimington Del. product ID Melinex HS-2).

The substrate may comprise papers and board made from wood pulp orcotton or synthetic wood free fibres. The paper/board may be coated,calendared or machine glazed.

The substrates being transparent filmic or non transparent substrateslike opaque plastic, paper including but not limited to banknote,voucher, passport, and any other security or fiduciary documents, selfadhesive stamp and excise seals, card, tobacco, pharmaceutical, computersoftware packaging and certificates of authentication, aluminium, andthe like.

In a preferred embodiment of the present invention the substrate is anon-transparent (opaque) sheet material, such as, for example, paper. Inanother preferred embodiment of the present invention the substrate is atransparent sheet material, such as, for example,polythyleneterephthalate.

The forming of an optically variable image on the substrate may comprisedepositing a curable compound, or composition on at least a portion ofthe substrate. The composition, generally a coating or lacquer may bedeposited by means of gravure, flexographic, ink jet and screen processprinting. The curable lacquer may be cured by actinic radiations,preferably ultraviolet (U.V.) light or electron beam. Preferably, thelacquer is UV cured. UV curing lacquers can be obtained from CibaSpecialty Chemicals. The lacquers exposed to actinic radiations orelectron beam used in the present invention are required to reach asolidified stage when they separate again from the imaging shim in orderto keep the record in their upper layer of the sub-microscopic,holographic diffraction grating image or pattern (OVI). Particularlysuitable for the lacquers compositions are chemistries used in theradiation curable industries in industrial coatings and graphic arts.Particularly suitable are compositions containing one or severalphoto-latent catalysts that will initiate polymerization of the exposedlacquer layer to actinic radiations. Particularly suitable for fastcuring and conversion to a solid state are compositions comprising oneor several monomers and oligomers sensitive to free-radicalpolymerization, such as acrylates, methacrylates or monomers or/andoligomers, containing at least one ethylenically unsaturated group.Reference is made to WO2008/061930.

The curable composition is preferably deposited by means of gravure orflexographic printing.

The curable composition is preferably curable by means of an ultraviolet(U.V.) light or an electron beam. The curable composition can becoloured.

The metallic ink may be applied to the substrate by means ofconventional printing press such as gravure, rotogravure, flexographic,lithographic, offset, letterpress intaglio and/or screen process, orother printing process.

In order that the hologram is clearly visible on both the first andsecond surface of a clear filmic substrate and the first surface of apaper substrate, preferably, the metallic pigment particles are printedin such a way as to align themselves such that they follow and conformto the contours of the diffraction grating.

To accomplish this alignment of metal pigment flakes to the contours ofthe diffraction grating the coating composition preferably has a verylow binder, a low pigment content and a medium pigment to binder ratioand/or very thin pigment particles.

The coating composition preferably comprises low solids, high viscositybinders. Preferably, the pigment to binder ratio is in the range of 2:1to 1:3 by weight. More preferably, the pigment to binder ratio is byweight in the range of 1.5:1 to 1:1, and even more preferably 1:1 to1.5:1. Most preferably the pigment to binder ratio is 1:3.

The binder may comprise any one or more selected from the groupcomprising nitro cellulose, vinyl chloride, vinyl acetate copolymers,vinyl, acrylic, urethane, polythyleneterephthalate, terpene phenol,polyolefin, silicone, cellulose, polyamide, polyester, rosin esterresins. The preferred binder is 50% nitrocellulose (ID nitrocelluloseDHL120/170 and nitrocellulose DLX30/50 supplied by Nobel Industries) 50%polyurethane (ID Neorez U335 supplied by Avecia). The solvents may beester/alcohol blends and preferably normal propyl acetate and ethanol ina ratio of 20:1 to 30:1.

The compositions of the present invention can be applied to opticallyvariable images (holograms) for use on substrates such as (securityproducts), including banknotes, credit cards, identification documentslike passports, identification cards, drivers licenses, or otherverification documents, pharmaceutical apparel, software, compact discs,tobacco packaging and other products or packaging prone tocounterfeiting or forgery, to protect them from fraudulent conversion,diversion or imitation.

The thickness of the pigment particles may be less than 10 nm. Thethickness of the pigment particles may be above 5 nm.

Preferably, the thickness of the pigment particles is between 7 and 9nm. More preferably, the thickness of pigment particle is about 8 nm.

Preferably, when the substrate carrying the metallised image or patternis subsequently over-laid onto printed pictures and/or text, or thesubstrate is pre-printed with pictures and/or text and the metallisedimage or pattern is deposited thereon those pre-printed features arevisible through the substrate and/or the metallic composition coateddiffraction grating or image.

The coating compositions of the present invention may be deposited on adiffraction grating disposed on a substrate such as a substantiallytransparent, translucent, or opaque substrate. The substrate maycomprise paper, filmic material or metal, such as aluminium.

The substrate may comprise polymeric compounds. The substrate maycomprise papers made from wood pulp or cotton or synthetic wood-freefibres.

The diffraction grating may be formed using any methods known to theskilled man such as those described in U.S. Pat. No. 4,913,858, U.S.Pat. No. 5,164,227, WO2005/051675 and WO2008/061930.

The coating composition of the present invention may be applied to thesubstrate by means of conventional printing press such as gravure,rotogravure, flexographic, lithographic, offset, letterpress intaglioand/or screen process, or other printing process.

Preferably, when the substrate carrying the enhanced diffractive imageor pattern is subsequently over-laid onto printed pictures and/or text,or the substrate is pre-printed with pictures and/or text and theenhanced diffractive image or pattern is deposited thereon, thoseprinted features are visible through the substrate and/or the metallicink coated diffraction grating or image.

By varying the film-weight and density of the deposited metallic ink,the transmission of light through the enhanced image can be adjusted toprovide a desirable range of visual effects.

The composition may further comprise modifying additives, for examplecolorants and/or suitable solvent (s).

Preferably, the resin maintains adhesion of the composition to thesurface of the diffraction grating.

Specific additives can be added to the composition to modify itschemicals and/or physical properties. Polychromatic effects can beachieved by the introduction of transparent organic pigments and/orsolvent soluble dyestuffs into the ink, to achieve a range of colouredshades.

The binder resins may be initially dissolved in the appropriatesolvent(s) to form liquid varnishes. These varnishes can then be blendedtogether with the metallic pigment and/or other components by means of ahigh-speed blender to produce the composition.

In accordance with a further aspect of the present invention, there isprovided a metallic ink comprising the aluminium flakes of the presentinvention and a binder. The ratio of pigment to binder is sufficientlyhigh as to permit the alignment of the aluminum particles to thecontours of a diffraction grating.

The aluminium particles may be prepared by any means known to theskilled man. Reference is made to GB-A-1,465,908 (U.S. Pat. No.4,116,710), U.S. Pat. No. 4,321,087, WO0024946, WO0024946, WO020090613,WO03046245, WO02/094945, WO06/021528, WO07/057,328 and WO2005/049745.Preferably, a 12-micron thick transparent carrier film such as

Polythyleneterephthalate obtained from DuPont Films Wilmington. Del.(Product ID Melinex HS-2) two metres wide is gravure coated with anacrylic resin isobutyl methacrylate obtained from DuPont (Product IDElvacite 2045) and dried by means of hot air. In a second operation theacrylic-coated film is deposition coated with aluminium by means of aroll to roll vacuum chamber. The deposition rate and thickness of thevaporised aluminium layer over the printed acrylic coating is accuratelycontrolled through continuos monitoring of the optical density duringmanufacture. The operating range of vacuum deposition may be in therange of 7 to 9 nm.

The optical density may be in the range of 0.16 to 0.24 as measured onthe McBeth densitometer. Preferably, the range is 0.18 to 0.22. Morepreferably, the optical density is 0.2 as measured on the McBethdensitometer.

The aluminium layer may be removed from the carrier film by means ofdesolving the acrylic supporting layer in a bath containing ethylacetate releasing the aluminium layer from the carrier film. Theresulting aluminium in the form of a coarse flake in the resin solutionmay then be washed in a multi stage centrifuging process to remove theacrylic resin. The coarse aluminium flakes are mixed with ethyl acetateand disintegrated by a high shear mixing process to produce a controlledparticle size distribution. The median particle diameter may be in therange of 5 to 12 microns the preferred range being 7 to 9 micronsdiameter as measured by a Coulter LS130 I. a. s. e. r. diffractiongranulometer.

The metallic ink of the present invention can also be used in theproduction of a hot stamping foil.

Accordingly, the present invention is also directed to a method ofproducing a hot stamping foil comprising the steps of:

(a) coating a carrier with a release coating,(b) applying a coating of a hard lacquer onto the release coating,(c) applying an ultraviolet primer coating onto the coating of the hardlacquer,(d) contacting at least a portion of the ultraviolet primer coating withoptically variable image (optically variable device) forming means;(e) depositing the metallic ink of the present invention to the UVprimer holding the transferred optically variable device either as awhole or in partial areas,(f) optionally printing subsequently process colours, and(g) applying a heat activated adhesive onto the layer obtained in stepe), or f).

Examples of the release compound are silica, microcrystalline wax, ricewax, oricuri wax, stearic acid esters, polyglycols, and metallic saltsof fatty acids.

Examples of the coating of the hard lacquer are polymethylmethacrylate,styrene acrylonitrile, polyethyleneterephthalate, nitrocellulose, ormixtures thereof. The coating of the hard lacquer affixed to saidrelease coating has in general a thickness in the range 0.25 microns to9 microns and has a glass transition temperature of at least 70° C.

Examples of the adhesive compound are vinyl alcohol, polyacrylates,polyalkacrylates, vinyl resins, polyvinyl acetate, cellulose resins,polyacrylamides, and ethylene/vinyl acetate copolymers.

Under the present invention a carrier film substrate having thethickness on the order of 12 microns to 75 microns and formed of asuitable plastic material such as a polyester, oriented polypropylene orother suitable material is coated with a release coating such as amicrocrystalline wax or a partially saponified montan wax or other waxbased coatings having a thickness in the range of 0.025 microns to 5microns and then has a coating of hard lacquer applied over the releasecoating in a thickness in the range of 0.25 microns to 10 microns. Thehard lacquer coating may be applied by a gravure roller following whichis dried an ultraviolet primer coating having the thickness in a rangeof 0.3 microns to 9 microns is then applied by means of a gravureroller. An optically variable device is contacted by the embossing shimhaving an optically variable device thereon and is transferred into thesurface of the UV lacquer and cured by UV light. A layer of the vacuummetallised aluminium ink is applied to the UV primer holding thetransferred optically variable device either as a whole or in partialareas. Subsequent process colours can be gravure printed.

The hot stamping foil obtained in the above process can be used to labelan article of manufacture. The method of labeling the article ofmanufacture comprises the steps of: contacting the heat activatedadhesive layer of the hot stamping foil obtained according to the aboveprocess with said article; hot stamping said hot stamping foil to causesaid heat activated adhesive layer to adhere to said article; andremoving the carrier of said hot stamping foil from said hard lacquerlayer.

The optically variable device (OVD) is, for example, an diffractiveoptical variable image (DOVI). The term “diffractive optical variableimage” as used herein may refer to any type of holograms including, forexample, but not limited to a multiple plane hologram (e.g.,2-dimensional hologram, 3-dimensional hologram, etc.), a stereogram, anda grating image (e.g., dot-matrix, pixelgram, exelgram, kinegram, etc.).

Examples of an optically variable image or device are holograms ordiffraction gratings, moire grating, etc. These optical microstructuredimages are composed of a series of structured surfaces. These surfacesmay have straight or curved profiles, with constant or random spacing,and may even vary from microns to millimetres in dimension. Patterns maybe circular, linear, or have no uniform pattern. For example a Fresnellens has a microstructured surface on one side and a pano surface on theother. The microstructured surface consists of a series of grooves withchanging slope angles as the distance from the optical axis increases.The draft facets located between the slope facets usually do not affectthe optical performance of the Fresnel lens.

Various aspects and features of the present invention will be furtherdiscussed in terms of the examples. The following examples are intendedto illustrate various aspects and features of the present invention, butnot to limited the scope of the present invention.

EXAMPLES Example 1

A 12-micron thick transparent carrier film, two meters wide, made ofpolythyleneterephthalate was obtained from ICI Films, Wilmington, Del.,USA (Melinex HS-2) is gravure coated with an acrylic resin, isobutylmethacrylate, obtained from DuPont (Elvacite 2045), and dried by meansof hot air. In a second operation the acrylic-coated film is depositioncoated with aluminium by means of a roll to roll vacuum chamber. Thedeposition rate and thickness of the vaporised aluminium layer over theprinted acrylic coating are accurately controlled through continuousmonitoring of the optical density during manufacture (The aluminum layeris vapor deposited at 8-9 nm measured by an IC/5 controller. Thecontroller for the aluminum layer is calibrated by a MacBeth TR927transmission densitometer with green filter). Several rolls weremetallised at different thicknesses of aluminium, in order to give flakeproducts of various thicknesses (see Table below).

Product d50 Thickness*) Optical Density**⁾ Product A) 7.0-11.0 μm 8-9 nm0.2 Comparative Product 8.5-11.5 μm  17 nm 0.7 B) (WO2005/049745)*)Flake thickness measured directly by sectioning and SEM, or via AtomicForce Microscopy. **⁾Optical density of the vaporised aluminium layerover the printed acrylic coating measured by the Macbeth Densitometer(X-Rite 301 Transmission Densitometer) during manufacture.

The aluminium layer is removed from the carrier film by means ofdissolving the acrylic supporting layer in a bath containing ethylacetate, releasing the aluminium layer from the carrier film. Theresulting aluminium, in the form of a coarse flake in the resinsolution, is then washed in a multi stage centrifuging process to removethe acrylic resin. The coarse aluminium flakes were mixed with ethylacetate and disintegrated by a high shear mixing process to produce acontrolled particle size distribution.

The suspension of vacuum metallised aluminium flakes (Product A), orComparative Product B)) in ethyl acetate is mixed with nitrocellulose,n-propylacetate and Dowanol PM to make a coating composition.

The coatings are printed on an RK proofer press over an aluminum foil,and white coated papered paper provided with a holographic image byapplying a clean UV curable varnish onto the corona treated substrateand embossing by using a shim and exposing to UV light. Product A) isextremely darker than Comparative Product B).

While Comparative Product B) shows bright OVD image and good rainboweffect, product A) is characterized by an extremely bright OVD image andextremely strong rainbow effect, high purity and contrast.

Application Example 1

1a) Varnish preparation: 14.3 g of nitrocellulose (DHM 10-25 IPA (NobelEntreprises, UK)) are slowly added to 85.7 g of ethylacetate (99-100%rein, Brenntag) in a 250 mL glass bottle and gently stirred untilcomplete dissolution at room temperature. Solid content measurements arethen performed and quantity of ethylacetate is adjusted to achieve avalue of 10% solid content in the varnish preparation.

1b) 2.25 g of the above prepared varnish, 1.23 g of nitrocellulose(DHX3/5 ETH (Nobel Enterprises, UK)) and 0.34 g of methacrylate resin(DEGALAN 4793-L (EVONIK Industries Degussa)) are mixed with 5.4 g ofethylacetate in a 25 mL glass bottle and slowly stirred with a Dispermatapparatus at 2000 rpm for 5 min. Addition of 0.8 g of the aluminiumflakes of example 1 and stirring with a Dispermat at 800 rpm for 10 minaffords a metallic ink which is printed by hand-coater (HC2, 12 μm wetfilm thickness) on contrast paper and transparent PET film.

Application Example 2

2.00 g of the varnish prepared in the application example 1a, 1.09 g ofnitrocellulose (DHX3/5 ETH (Nobel Enterprises, UK)) and 0.30 g ofmethacrylate resin (DEGALAN 4793-L (EVONIK Industries Degussa)) aremixed with 4.8 g of ethylacetate in a 25 mL glass bottle and slowlystirred with a Dispermat apparatus at 2000 rpm for 5 min. Addition of1.8 g of the aluminium flakes of example 1 and stirring with a Dispermatat 800 rpm for 10 min affords a metallic ink which is printed byhand-coater (HC2, 12 μm wet film thickness) on contrast paper andtransparent PET film.

Application Example 3

1.76 g of the varnish prepared in application example 1a, 0.96 g ofnitrocellulose (DHX3/5 ETH (Nobel Enterprises, UK)) and 0.26 g ofmethacrylate resin (DEGALAN 4793-L (EVONIK Industries Degussa)) aremixed with 4.2 g of ethylacetate in a 25 mL glass bottle and slowlystirred with a Dispermat apparatus at 2000 rpm for 5 min. Addition of2.8 g of the aluminium flakes of example 1 and stirring with a Dispermatat 800 rpm for 10 min affords a metallic ink which is printed byhand-coater (HC2, 12 μm wet film thickness) on contrast paper andtransparent PET film.

Application Example 4

1.24 g of the varnish prepared in application example 1a, 0.68 g ofnitrocellulose (DHX3/5 ETH (Nobel Enterprises, UK)) and 0.18 g ofmethacrylate resin (DEGALAN 4793-L (EVONIK Industries Degussa)) aremixed with 2.98 g of ethylacetate in a 25 mL glass bottle and slowlystirred with a Dispermat apparatus at 2000 rpm for 5 min. Addition of3.8 g of the aluminium flakes of example 1 and stirring with a Dispermatat 800 rpm for 10 min affords a metallic ink which is printed byhand-coater (HC2, 12 μm wet film thickness) on contrast paper andtransparent PET film.

Coloristic measurements are performed with an X-Rite SP68 (d/8)spectrophotometer on each white and black part of the contrast paper.CieLab L*, a*, b* coordinates are given for a D65 standard illuminantand 10° observer angle.

Application Pigment/ Contrast Example Binder Paper L* a* b* 1 0.12/1white 48.4 0.6 0.1 2 0.30/1 white 48.1 0.9 1.3 3 0.54/1 white 50.9 2.37.6 4 1.03/1 white 56.2 2.2 11.0 1 0.12/1 black 48.7 −0.3 −0.7 2 0.30/1black 48.2 0.2 0.0 3 0.54/1 black 49.5 2.1 5.6 4 1.03/1 black 54.1 2.49.8

Observation of the prints on transparent PET film reveals a gold shadeupon direct viewing and a darker bronze-gold shade upon reverse viewing(through PET film).

Application Example 5 Pigment:Binder Ratio 1:5, Total Non-VolatileContent 2.4%

10% Cellulose Acetate Butyrate (CAB) Solution:

CAB (e.g. CAB 381-20 supplied by 10% by weight Eastman) n-Butyl acetate45% by weight Xylene (mixed isomers) 45% by weight Σ 100% by weight 

25% Polyester Resin Solution:

Polyester resin (e.g. Setal 173-VS-60 42% by weight supplied by NuplexResins) n-Butyl acetate 29% by weight Xylene (mixed isomers) 29% byweight Σ 100% by weight 

4.0 g of a 10% slurry of the aluminium flakes of example 1 are added to10.0 g of a 10% CAB solution, 4.0 g of a polyester resin solution, 41.0g n-butyl acetate and 41.0 g xylene (mixed isomers) and mixed under lowshear conditions (magnetic stirrer or low speed propeller stirrer) untila fully ‘wetted-out’ homogeneous liquid slurry is obtained.

The sample is best applied onto a substrate at full opacity (appliedover black & white substrate or with black & white marker to ensureopacity) by use of pneumatic or electrostatic spray application(draw-down, pour or other conventional application techniques could alsobe used with some variation in appearance) and then following a‘flash-off’ period either at room temperature or under mild forcedconditions (flash-off period is the time allowed for solvent to bereleased from the coating prior to further processing and can be with orwithout curing of the film), a 1-component stoving clearcoat is applied(use of 1-component or 2-component clearcoats are both feasible) andagain after a further ‘flash-off’ period the whole article is cured atelevated temperature to bring about the final film properties.

Application Example 6 Pigment:Binder Ratio 1:3, Total Non-VolatileContent 2.4%

6.0 g of a 10% slurry of the aluminium flakes of example 1 is added to9.0 g of a 10% CAB solution, 3.6 g of a polyester resin solution, 40.7 gn-butyl acetate and 40.7 g xylene (mixed isomers) and mixed under lowshear conditions (magnetic stirrer or low speed propeller stirrer) untila fully ‘wetted-out’ homogeneous liquid slurry was obtained. Applicationconditions are the same as for application example 5.

Application Example 7 Pigment:Binder Ratio 1:1, Total Non-VolatileContent 2.4%

12.0 g of a 10% slurry of the aluminium flakes of example 1 are added to6.0 g of a 10% CAB solution, 2.4 g of a polyester resin solution, 39.8 gn-butyl acetate and 39.8 g xylene (mixed isomers) and mixed under lowshear conditions (magnetic stirrer or low speed propeller stirrer) untila fully ‘wetted-out’ homogeneous liquid slurry is obtained. Applicationconditions are the same as for application example 5.

Application Example 8 Pigment:Binder Ratio 1:0.5, Total Non-VolatileContent 2.4%

16.0 g of a 10% slurry of the aluminium flakes of example 1 are added to4.0 g of a 10% CAB solution, 1.60 g of a polyester resin solution, 39.2g n-butyl acetate and 39.2 g xylene (mixed isomers) and mixed under lowshear conditions (magnetic stirrer or low speed propeller stirrer) untila fully ‘wetted-out’ homogeneous liquid slurry was obtained. Applicationconditions are the same as for application example 5.

Coloristic measurements are performed with a Minolta CM-2600D (d/8)spectrophotometer. Opacity is ensured by comparing DE values whenmeasured over the white and black substrate. CieLab L*, a*, b* valuesare given with D65/10 illumination.

Application Pigment/ Opacity over Example Binder black & white L* a* b*5 1:5 Yes 52.2 0.8 3.7 6 1:3 Yes 55.5 1.2 4.9 7 1:1 Yes 61.4 1.4 8.8 8  1:0.5 Yes 66.3 0.9 8.8

1. An aluminium flake having a thickness above 5 nm and below 10 nm. 2.The aluminium flake according to claim 1 having a thickness between 7and 9 nm.
 3. The aluminium flake according to claim 1, wherein the meanparticle diameter D₅₀ of the aluminum particles is in the range of 8.0to 15.0 μm.
 4. A coating composition, comprising the aluminium flakesaccording to claim
 1. 5. The coating composition according to claim 4,comprising the aluminium flakes according to claim 1 and a binderwherein the ratio of pigment to binder is sufficiently high as to permitthe alignment of the pigment particles to the contours of thediffraction grating.
 6. The coating composition according to claim 5,wherein the composition further comprises a solvent.
 7. The coatingcomposition according to claim 6, wherein the solvent comprises any oneor more of the group comprising an ester, a ketone, an alcohol, anaromatic hydrocarbon and water.
 8. The coating composition according toclaim 6, wherein the binder comprises any one or more selected from thegroup comprising nitrocellulose, ethyl cellulose, cellulose acetate,cellulose acetate propionate (CAP), cellulose acetate butyrate (CAB),alcohol soluble propionate (ASP), vinyl chloride, vinyl acetatecopolymers, vinyl acetate, vinyl, acrylic, polyurethane, polyamide,rosin ester, hydrocarbon, aldehyde, ketone, urethane,polythyleneterephthalate, terpene phenol, polyolefin, silicone,cellulose, polyamide, polyester and rosin ester resins.
 9. Thecomposition as claimed claim 4 for use in coating an optically variableimage (hologram).
 10. A metallic ink comprising the aluminium flakesaccording to claim 1 and a binder.
 11. The metallic ink of claim 10,wherein the ratio of pigment to binder is sufficiently high to permitthe alignment of the pigment particles to the contours of a diffractiongrating
 12. A method for forming an optically variable image (anoptically variable device) on a substrate comprising the steps of: a)forming an optically variable image (OVI) on a discrete portion of thesubstrate; and b) depositing the metallic ink according to claim 11 onat least a portion of the diffraction grating.
 13. The method accordingto claim 12, comprising the steps of: a) applying a curable compound toat least a portion of the substrate; b) contacting at least a portion ofthe curable compound with OVI forming means; c) curing the curablecompound and d) depositing the metallic ink on at least a portion of thecured compound, said metallic ink comprise an aluminium flake having athickness of 5 nm to 10 nm and a binder.
 14. A (security) productobtainable by using the method as claimed in claim
 12. 15. A method ofproducing a hot stamping foil comprising the steps of: (a) coating acarrier with a release coating, (b) applying a coating of a hard lacqueronto the release coating, (c) applying an ultraviolet primer coatingonto the coating of the hard lacquer, (d) contacting at least a portionof the ultraviolet primer coating with optically variable image(optically variable device) forming means; (e) depositing the metallicink according to claim 11 to the UV primer holding the transferredoptically variable device either as a whole or in partial areas, (f)optionally printing subsequently process colours, and (g) applying aheat activated adhesive onto the layer obtained in step e), or f).
 16. Amethod of labeling an article of manufacture, the method comprising thesteps of: contacting the heat activated adhesive layer of the hotstamping foil obtained according to claim 15 with said article; hotstamping said hot stamping foil to cause said heat activated adhesivelayer to adhere to said article; and removing the carrier of said hotstamping foil from said hard lacquer layer.